Conventional engines for propelling automobiles are typically the spark ignition type and, to a lesser extent, the compression ignition diesel type. Both types demonstrate a less than optimum fuel economy at varying road loads. Since automotive use is rarely at optimum load, economy is compromised. Free piston engines have shown superior indicated thermal efficiency; however, the methods of power conversion yield poor efficiency and no overall advantage.
The internal combustion engine of the present invention is a free piston engine operating on a Otto cycle with autoignition. Free piston engines are well known including engines employing opposed pistons operating within a cylinder. The pistons are driven initially toward each other in the cylinder to compress an injected fuel charge to the condition of autoignition. The resulting combustion forces the pistons away from each other. Energy is extracted from the moving piston for external use and the pistons are driven back toward each other by a bounce action within the cylinders. [sometimes pneumatic spring driven and sometimes hydraulic spring driven ]In the known prior art free piston engines, the pistons continue to oscillate within the cylinder without dwell.
The free piston engine of the present invention differs in one major aspect from the prior art by including a brake system to provide a controlled dwell between cycles of the piston whereby the engine is controlled to cycle or "pulse" only when the energy from the prior pulse has been used by the energy demand system. The pulse rate of the engine varies directly with load. The combustion conditions are constant regardless of pulse rate and are optimized for maximum fuel economy. Furthermore, in the system herein disclosed of an engine and an energy demand system, the energy storage system is quite small since only cyclic pulse energy is stored. The free piston engine and energy demand system thus have a high power to weight ratio.